Arduino Board Getting Too Hot: Need Help Diagnosing Issue (Schematic Provided)

Question

I’m encountering an issue where one of my Arduino boards, specifically “Arduino 1” in the schematic attached below, is getting very hot shortly after powering up. I’m concerned that something in my circuit might be causing this, but I’m unsure what it could be.

Project Details:

• Arduino Board: Arduino Uno R3 (labeled “Arduino 1” in the schematic)
• Power Source: 12V DC input connected to the barrel jack.
• Connected Components:
  • A relay (Finder 36.11.9.003.4011)
  • A Peltier element controlled via a MOSFET (IRLZ44N) with a heatsink
  • A MAX31865 sensor interface
  • An LED panel connected through a DD-10-10P-PWM module
  • Some other standard connections like resistors, capacitors, and LEDs

Symptoms:

• “Arduino 1” gets noticeably hot, particularly around the voltage regulator and the microcontroller, within a couple of minutes after powering up.
• The rest of the components seem to work as expected.

What I’ve tried:

• Double-checked all wiring for possible shorts or incorrect connections.
• Disconnected peripherals one by one to isolate the problem, but the Arduino still gets hot.

Attached Schematic:

Questions:

1. Is a specific component or connection in the schematic causing this issue?
2. I am still determining my design, which uses common ground for power and digital circuitry. Can this be problematic?

I’d greatly appreciate any insights or suggestions.

Answer 1

The first thing of note is that you're trying to drive a 3V relay directly from the 5V output pin of the Arduino.

From the relay datasheet, the relay coil is 25R:

With a 5V supply, you'd be trying to drive 200mA from the IO pin which is far too high for the Atmega to cope with. Use a transistor to drive the coil to avoid damage. It also exceeds the relay coil ratings.

Note that the 3.3V supply from the Arduino shouldnt be used for driving the relay with the transistor as it likely won't be able to supply enough current. Use the 12V rail and a resistor of 75R to drop the voltage to an appropriate level for the relay you have (see below for why not to use the 5V rail), or use the 12V variant from that series if you have one.

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The side effect of driving this relay is that you are powering it via the linear regulator on the Arduino. With a 12V input the power dissipation in that regulator will be 7V times any current you draw. If you did manage 200mA from the IO pin, that would equate to 1.4W dissipation in the regulator - that will cause it to heat up considerably with no additional cooling.

From experience the IO pins generally top out a 90mA due to internal resistance, but that is still 0.63W due to the relay coil alone. Add on the MCU and other gubbins and you're likely still drawing over a watt, which will still heat up the regulator.

Depending on the temperature, this may be normal/acceptable level of heating. You'd have to check the temperature rating on the regulator on the Arduino.

Answer 2

Adding to Tom Carpenter's answer: Arduino boards have small regulators so you should take care to minimise the 5V current draw or add your own 5V regulator to your board, possibly with a heatsink. I'm fairly sure the Arduino primary rail is 5V and the 3.3V is a secondary, low current, output.
The pic (from the web) shows in general how to use a transistor to drive your relay - the flyback (or freewheel) diode is especially important. A series resistor will work but will dissipate 1.8W however you can connect the relay directly to 12V if you PWM it with a 25% duty cycle. Just ensure your software is reliable and doesn't lock up with the transistor on!